Hundreds of defendants sitting in prisons nationwide have been convicted with the help of an FBI forensic tool that was discarded more than two years ago. But the FBI lab has yet to take steps to alert the affected defendants or courts, even as the window for appealing convictions is closing, a joint investigation by The Washington Post and "60 Minutes" has found.

The science, known as comparative bullet-lead analysis, was first used after President John F. Kennedy's assassination in 1963. The technique used chemistry to link crime-scene bullets to ones possessed by suspects on the theory that each batch of lead had a unique elemental makeup.

In 2004, however, the nation's most prestigious scientific body concluded that variations in the manufacturing process rendered the FBI's testimony about the science "unreliable and potentially misleading." Specifically, the National Academy of Sciences said that decades of FBI statements to jurors linking a particular bullet to those found in a suspect's gun or cartridge box were so overstated that such testimony should be considered "misleading under federal rules of evidence."

A year later, the bureau abandoned the analysis.

But the FBI lab has never gone back to determine how many times its scientists misled jurors. Internal memos show that the bureau's managers were aware by 2004 that testimony had been overstated in a large number of trials. In a smaller number of cases, the experts had made false matches based on a faulty statistical analysis of the elements contained in different lead samples, documents show.

"We cannot afford to be misleading to a jury," the lab director wrote to FBI Director Robert S. Mueller III in late summer 2005 in a memo outlining why the bureau was abandoning the science. "We plan to discourage prosecutors from using our previous results in future prosecutions."

Despite those private concerns, the bureau told defense lawyers in a general letter dated Sept. 1, 2005, that although it was ending the technique, it "still firmly supports the scientific foundation of bullet lead analysis." And in at least two cases, the bureau has tried to help state prosecutors defend past convictions by using court filings that experts say are still misleading. The government has fought releasing the list of the estimated 2,500 cases over three decades in which it performed the analysis.

For the majority of affected prisoners, the typical two-to-four-year window to appeal their convictions based on new scientific evidence is closing.

Dwight E. Adams, the now-retired FBI lab director who ended the technique, said the government has an obligation to release all the case files, to independently review the expert testimony and to alert courts to any errors that could have affected a conviction.

"It troubles me that anyone would be in prison for any reason that wasn't justified. And that's why these reviews should be done in order to determine whether or not our testimony led to the conviction of a wrongly accused individual," Adams said in an interview. "I don't believe there's anything that we should be hiding."

The Post and "60 Minutes" identified at least 250 cases nationwide in which bullet-lead analysis was introduced, including more than a dozen in which courts have either reversed convictions or now face questions about whether innocent people were sent to prison. The cases include a North Carolina drug dealer who has developed significant new evidence to bolster his claim of innocence and a Maryland man who was recently granted a new murder trial.

Documents show that the FBI's concerns about the science dated to 1991 and came to light only because a former FBI lab scientist began challenging it.